A diverse community of commensal bacteria colonizes the mammalian gut. Interactions between the commensal microflora and the gut immune system are critical for establishing a proper balance between immune host defense mechanisms and tissue health. Although a few studies have reported the presence of fungal DNA in human and murine feces, gut fungi have been largely overlooked and their interactions with the gut immune system have not been investigated. We have recently reported an abundant and highly diverse population of fungal species (gut mycobiome) to be a significant component of the murine intestinal microbiota. We found that gut fungi interact with the immune system through the innate immune receptor Dectin-1 and that mice lacking Dectin-1 exhibit increased susceptibility to chemically-induced colitis, which is a result of altered responses to indigenous fungi. However not all fungi are bad. We found that a mouse commensal strain of Saccharomycopsis fibuligera (S. fibuligera ID1) was protective against intestinal inflammation in a DSS- induced model of colitis. Certain species of commensal bacteria have long been known to modulate mucosal immune responses, dictate the balance between Th1, Th2, Th17 and Treg cells and have been widely used for prevention and treatment of intestinal disorders. Only a few studies have reported that fungi can have similar protective properties, although the mechanism behind such protection is largely unknown. This proposal focuses on defining protective gut fungi and their mechanisms of protection by studying their interaction with bacteria and the intestinal epithelium as well as with the innate and adaptive immune systems in the gut. We hypothesize that commensal fungi interact with gut bacteria and with the mucosal immune system to induce tolerogenic mechanisms, leading to suppression of intestinal inflammation and colitis. We will test our hypothesis in three specific aims. In Specific Aim 1 we will evaluate commensal and related fungi for protective properties using a mouse model of DSS induced colitis. In Specific Aim 2 we will extend the analysis to look at protection by S. fibuligera ID1 ad other fungi in models targeting different arms of colitis etiology including T cell-mediated and infectious microbe-mediated disease. In Specific Aim 3 we will explore mechanisms of protection including fungal-induced alterations in the bacterial microbiome, interaction of gut fungi with the intestinal epithelium and the innate immune system, and regulation of T cell bias in the intestine.